The invention relates to a regenerated data signal generation apparatus for use in a wireless receiving system, and more specifically, the invention is applicable to a regenerated data signal generation apparatus wherein a signal based on a received signal is binarized (sliced) by use of a predetermined DC level, and a regenerated data signal is generated on the basis of the signal as binarized.
With a receiver of a wireless communications system, a regenerated data signal is generated by binarizing (slicing) a detected signal (demodulated signal) obtained by demodulating a received signal at a predetermined reference level. The DC potential of the detected signal can undergo deviation for various reasons. For example, with a radio receiver receiving a FSK (Frequency Shift Keying) signal, the DC potential of a signal (detected signal) demodulated from the received signal can undergo deviation (offset deviation) due to effects of difference between signal frequency of the FSK signal and pre-specified frequency of a carrier wave, and so forth. Accordingly, in the process of generating the regenerated data signal, there is the need for compensating for the offset deviation occurring to the detected signal in order to obtain an accurate regenerated data signal.
Various techniques for compensating for the offset deviation of the detected signal have been conventionally proposed. For example, with a demodulator disclosed in U.S. Pat. No. 6,104,238, deviation in DC potential outputted from a detection circuit is suppressed by smoothing detection signals to thereby vary the center frequency thereof.
Further, with a data slicer disclosed in U.S. Pat. No. 5,412,692, an intermediate potential between the maximum level potential and the minimum level potential, detected from a detection output signal, is detected, and the detection output signal is binarized (sliced) by use of an intermediate potential level as a reference level. As a result, the reference level (the intermediate potential level) used for binarization of a detected signal will come to respond to deviation of the detection output signal, so that offset deviation of the detection output signal is compensated for.
Now, with the wireless communications system, a unit (for example, a frame) of a transmission signal generally comprises a data text part and a preamble part appended in front of the data text par. The preamble part is fixed in a predetermined pattern for every wireless communications system. The data text part is a part corresponding to data to be actually transmitted, and its pattern is dependent on the data to be transmitted. Accordingly, the data text part can include a pattern in which either low level bits or high level bits are continuous (successive identical codes pattern). In the case of the wireless communications system, there is specified a pattern length (tolerance for successive identical codes) of the successive identical codes pattern, ensuring demodulation without an error.
The pattern of the preamble part is decided upon for every wireless communications system as described in the foregoing, and from the viewpoint of efficiency of information transmission, the pattern length of the preamble part not containing objective data is preferably as short as possible. In the case of compensating for offset deviation of DC potential at the preamble part having such a short pattern length during the process of generating a regenerated data signal, operation for compensation needs to respond to deviation of DC potential of the signal at high speed.
Further, there can occur a case where a radio receiver receives transmission signals having a makeup of the above-described units (frames) in burst. For example, with a radio receiver (base station, mobile station) of a certain type of wireless communications system (mobile communications system), a transmitting condition and a receiving condition are on a time division basis (TDD: Time Division Duplex), and further, there is a case where a pause condition (neither the transmitting condition nor the receiving condition although a power supply voltage is applied to transmitting and receiving units) exists between changeovers besides the case where the transmitting condition and the receiving condition are changed over in succession. In such a case, when the radio receiver is changed over to the receiving condition, transmission signals arrive at the radio receiver in burst, whereupon DC potentials of detected signals at that point in time undergo dynamic deviation.
In such a case where changeover takes place from the pause condition to the receiving condition, the detected signals whose DC potentials undergo dynamic deviation rapidly rise, and consequently, particularly high speed is required of operation for compensation of deviation in DC potential at the preamble part of the detected signals.
Meanwhile, since the data text part is a part corresponding to the objective data, it is important in the process of generating a regenerated signal that the data text part satisfies the tolerance for successive identical codes, specified by the wireless communications system, and is insusceptible to effects of noises while it is stable in operation. These properties are generally contradictory to the operation for compensation of deviation in DC potential at high speed. Accordingly, with a regenerated data signal generation apparatus ensuring that the data text part is insusceptible to the effects of noises and stable in operation while satisfying the tolerance for successive identical codes, high-speed of operation for compensation of deviation in DC potential at the preamble part will be sacrificed.
With the demodulator disclosed in U.S. Pat. No. 6,104,238, for example, time required for compensation of deviation in DC potential is the sum of time required for smoothing the detection signals and absolute delay time of a channel selection filter and a detection circuit, so that it is difficult to effect high-speed compensation of deviation in DC potential in a circuit employing a high-order filter. Further, with the data slicer disclosed in U.S. Pat. No. 5,412,692, there is the need for reducing the time constant of an integrating circuit for detection of the maximum level potential and the minimum level potential as detected from the detection output signal in order to implement high-speed compensation of deviation in DC potential, however, in such a case, there occurs deterioration in the tolerance for successive identical codes. Accordingly, the time constant of the integrating circuit can not be reduced in any range other than that within which it is possible to secure a condition where the tolerance for successive identical codes, as specified by a wireless communications system, is satisfied.